Transcranial Magnetic Stimulation (TMS) is a technique used to induce neuronal depolarization in specific brain regions. This is achieved simply by generating large, pulsed, electromagnetic fields with a coil probe designed for that specific purpose. These induced fields produce currents in the neuronal tissue that result in neuronal depolarization. TMS is non-invasive, quickly repeatable and the risks to the research subjects or patients are minimal. As a device that uses magnetically-induced electrical currents to modulate the brain, the advent of TMS in the clinical setting appeared to offer a more precise means of delivering current to the brain than the existing electroconvulsive therapies. While TMS has been found to hold great potential as both a research tool and a therapeutic treatment for mental illness, the technology suffers from (1) poor targeting capabilities (the stimulated areas typically include large regions where no stimulation is desired) and (2) the lack of an adequate method to quantitatively measure the actual stimulation delivered to the subject. These two issues are strongly related to each other in that, in order to improve the targeting capabilities of TMS devices, an accurate measurement of the electromagnetic fields is needed. In this proposal we aim to develop a novel methodology to map and quantify TMS induced fields using magnetic resonance imaging (MRI) techniques. Furthermore, we aim to develop a fast computational algorithm that will predict the stimulation fields on an individual basis. We will compare our MRI based measurements and our computational predictions for cross-validation purposes. With this framework in place, we will use the computational model in conjunction with standard engineering optimization techniques to improve the design of TMS coils and stimulation pulses. The broader objective of this proposal is to develop a more region-specific and quantifiable transcranial magnetic stimulation system. In the long term, we hope to use this technology to fully understand the therapeutic effects of TMS and thus be able to optimize therapeutic treatment parameters. An additional benefit of the improved targeting capabilities we hope to achieve will be a significant improvement in our ability to use TMS as a research tool in psychiatry and cognitive psychology.